Technical Field
The following disclosure relates to an acoustic structure having a cavity in which sound waves propagate.
Description of Related Art
One example of the acoustic structure is a back chamber of a speaker. In an instance where a sound wave having a specific frequency propagates in the cavity of such an acoustic structure, there is generated a standing wave by superposition of the sound wave and reflected waves on a wall surface that defines the cavity, thereby causing a risk of a disturbance in frequency characteristics of the acoustic structure. In an instance where the frequency of the standing wave falls within a reproduction range of the speaker (i.e., a frequency range defined by the lower limit and the upper limit of frequencies of sounds represented by audio signals input to the speaker), peaks and dips in accordance with the frequency of the standing wave appear in the frequency characteristics of the speaker which should be flat. In view of this, there have been proposed various techniques of suppressing the disturbance in the frequency characteristics that arises from the standing wave. For instance, the following Non Patent Literatures 1 and 2, U.S. Pat. No. 4,127,751, and JP-56-140799A propose such techniques.
Non Patent Literatures 1 and 2 disclose an acoustic structure (as a back chamber of a speaker) in the form of a conical tapering tube, for suppressing reflection of the sound waves and accordingly suppressing generation of the standing wave. The acoustic structure is formed as the tapering tube for the purpose of avoiding generation of portions in the cavity at which acoustic impedance abruptly changes, in view of the fact that the reflection of the sound waves occurs at those portions at which acoustic impedance abruptly changes. U.S. Pat. No. 4,127,751, and JP-56-140799A propose a technique of suppressing generation of the standing wave by providing a sound absorber in the cavity of the acoustic structure.    Non Patent Literature 1: Bowers-Wilkins, retrieved on Apr. 21, 2015, [online], <URL:http://www.bowers-wilkins.jp/Discover/Discover/Technologies/nautilus-tapering-tubes.html>    Non Patent Literature 2: Norh, retrieved on May 26, 2015, [online], <URL:http://www.norh.com/Norh_Loudspeakers/Technlogy.html>    Patent Literature 1: U.S. Pat. No. 4,127,751    Patent Literature 2: JP-56-140799A    Patent Literature 3: JP-2014-175807A